1. Field of the Invention
The present invention relates to a method for determining the pH value of a liquid, a method for monitoring the pH value of a liquid, a method for obtaining drinking water from a fuel for aircraft and a sensor arrangement for determining the pH value of a liquid.
2. Background Information
In addition to electric power, fuel cells that are operated with kerosene also produce deoxygenated air and water. The water produced thus must be mineralized in order to purify the water to make drinking water. The mineralization process can be monitored using a pH sensor, which, by way of the pH value of the water, determines whether the mineralization of the water from the fuel cell is already sufficient for the water to have drinking-water quality.
The pH value is usually measured by means of ion-selective electrodes such as e.g. a glass electrode 200, shown in FIG. 16, or by means of ion-sensitive field effect transistors 201, as shown in FIG. 17. If a glass electrode 200 as shown in FIG. 16 is used, a reference electrode 202, which is usually immersed in a 0.1 molar KCl solution 204, is in electric contact via a diaphragm 206 to a measurement solution, the pH value of which is to be determined. Here, although the diaphragm 206 enables the electric contact with the measurement solution, it prevents mass transfer between the measurement solution and the KCl solution 204 to the greatest possible extent.
Arranged in the interior of the glass electrode 200 there is a measurement electrode 208, which is usually immersed into a phosphate buffer solution set to pH=7 as inner solution 210. The measurement electrode 208 has a conductive connection to the measurement solution via a very thin glass membrane 212. There are freely mobile sodium and lithium ions in the glass membrane 212; the glass membrane 212 is impermeable to hydrogen ions.
Upon contact with the aqueous solution, the glass membrane 212 starts to swell on the surface and hydrogen ions can take up lattice sites on oxygen anions of the glass membrane 212. In the case of a low pH value, this pushes the sodium and lithium ions back into the membrane, and so a modified potential can be measured at the measurement electrode 208. By contrast, in the case of a high pH value, a potential with opposite sign is created because the process runs in the other direction.
As an alternative to the glass electrode 200, use can be made of an ion-sensitive field effect transistor 201, which, as a simple transistor, is provided with a voltage source 216 and a voltage drain 218, which are separated from one another by an insulator 220. Hydrogen ions 222 from the measurement solution, the pH value of which is to be measured, are deposited on the insulator 220, which is usually formed by an oxide lattice. In the process, a positive voltage is created on the outer side of the insulator 220, which is mirrored on the inner side of the insulator 220. This means that a negative voltage 224 is created there. The higher the pH value of the measurement solution is, i.e. the fewer H+ ions are present in the measurement solution, the fewer hydrogen ions 222 are deposited on the insulator 220 and the negative voltage which flows between voltage source 216 and voltage drain 218 reduces. Conversely, the voltage increases between the voltage source 216 and the voltage drain 218 if there is a lower pH value, because more hydrogen ions 222 can be deposited on the insulator 220 in this case.
The two measurement methods, shown in FIGS. 16 and 17, for determining the pH value of a measurement solution are both disadvantageous in that they need to be immersed into the medium to be measured in order thereby to come into contact therewith. In doing so, there is a risk of dirtying the measurement solution and also the respective sensor. Furthermore, the sensors must be recalibrated at regular intervals in the case of electrochemical pH measurements in order to obtain reliable and reproducible measurement results in respect of the pH value.